In operating an elevator door system, it is important to know if the doors are not operating properly because something is blocking the doors from closing or opening. One way to do this is to measure the motor current; if an object is stuck in the doors, the motor current increases to supply enough current, and therefore door motor torque, to overcome the obstruction and nudge the doors closed. Typically, such a motor is an induction motor fed by a three-phase AC current provided by an inverter, itself responsive to a DC bus voltage, across a DC link. One way to determine the load current in the motor is to measure a current in the DC link, and particularly the DC component, which is proportional to the motor load current. For the protection of DC link current sensor circuitry it is also important that whatever DC link current sensing circuit is connected to the elevator control system be insulated from the DC bus voltage circuitry.
A straightforward method to obtain the DC component of the DC link current is to connect the primary side of a DC current transformer to the DC link and obtain the DC component of the DC link current straight off the secondary side. DC current transformers are expensive, however.
The next possible solution would be to connect an AC current transformer in the same way. But, given that the current in the DC link of an inverter for an elevator drive is on the order of ten amperes, a suitable transformer is large and therefore expensive, though not as expensive as a DC current transformer. Such a current transformer is difficult to install because of its size and requires heavy-duty means for securing it, involving still more cost.